1. Field of the Invention
The present invention relates to a femoral stem for artificial coxal articulation, and more particularly a femoral stem for artificial coxal articulation, in which portions of the femoral stem interfering with and contacting an insertion in the joint movement of the femoral stem are removed from the femoral stem so as to maximize the range of joint movement thereof while maintaining the cross sectional area thereof, and a method for processing a neck portion of the femoral stem.
2. Description of the Related Art
Generally, as shown in FIG. 1, coxal articulation refers to articulation for connecting the femur 200 and the pelvis 100 of a human body, and plays the primary role in sitting or standing. The coxal articulation is damaged by various diseases or external injuries. In order to cure the damaged coxal articulation 300, an artificial coxal articulation is applied through a surgical operation.
As shown in FIG. 2, the artificial coxal articulation comprises an acetabulum cup 4 fixed to the acetabulum 300 of the pelvis 100, an artificial bone head 3 fixedly inserted into the femur 200, and a femoral stem 1.
An insertion 2 having a hemispheric shape made of ceramic or high molecular weight polyethylene is fixed into the acetabulum cup 4, and the artificial bone head 3 having a spherical shape made of ceramic or metal is formed on one end of the femoral stem 1. The artificial bone head 3 is connected to the insertion 2 such that the artificial bone head 3 is rotated against the insertion 2 according to the movements of the femur 200 and the femoral stem 1.
The most important point of the above-described artificial coxal articulation is how to perform the rotational movement of the artificial coxal articulation in a range similar to the joint movement of coxal articulation of the human body.
The joint movement of the coxal articulation is divided into flexion in which the femur moves in the front direction of the pelvis and extension in which the femur moves in the reverse direction, as shown in FIG. 3a, adduction in which the femur moves inside the pelvis and abduction in which the femur moves outside the pelvis, as shown in FIG. 3b, and internal rotation in which the femur rotates in the inner direction of the pelvis and external rotation in which the femur rotates in the reverse direction, as shown in FIG. 3c. 
That is, as shown in FIG. 4, the flexion and extension respectively refer to the movements of the femoral stem 1 in the front and rear directions of the insertion 2, the adduction and abduction respectively refer to the movements of the femoral stem 1 to the inside and outside of the insertion 2, and the internal and external rotations respectively refer to the rotations of the femoral stem 1 in the inner and outer directions of the insertion 2.
Accordingly, the range of rotation of the artificial coxal articulation is determined by a range of the rotational movement of the femoral stem 1, i.e., an angle of rotation of the femoral stem 1. When the femoral stem 1 is manufactured, in order to minimize interference of the femoral stem 1 with the insertion 2, the diameter of a neck portion of the femoral stem 1 is decreased.
As shown in FIG. 2, the femoral stem 1 comprises a femoral stem main body 11 directly and fixedly inserted into the femur, a cutting conical portion 13 formed on one end of the femoral stem main body 11 so that the artificial bone head 3 is inserted into the cutting conical portion 13, and a neck portion 12 formed between the cutting conical portion 13 and the femoral stem main body 11.
Here, the neck portion 12 of the femoral stem 1 contacts the insertion 2 by the joint movement of the femoral stem 1. Accordingly, the smaller the diameter of the neck portion 12, the larger the range of movement of the femoral stem 1. However, when the diameter of the neck portion 12 is excessively small, the neck portion 12 is broken. Accordingly, the diameter of the neck portion 12 has a limit.
Recently, in order to overcome the above limit, a method, in which the joint movement of the femoral stem 1, particularly the range of the flexion of the femoral stem 1, is increased while the limit of the diameter of the neck portion 12 is maintained, has been proposed. For this reason, as shown in FIG. 5, one example of a sectional structure of a neck portion of the conventional femoral stem 1 is proposed.
As described above, the femoral stem 1 for artificial coxal articulation comprises the femoral stem main body 11, the cutting conical portion 13, and a neck portion 12 a having a circular cross section. As shown in FIG. 5, the neck portion 12 a has a limit diameter, and includes cutting planes 14 symmetrically formed at both sides thereof directly contacting the insertion 2 when the femoral stem 1 performs joint movement.
That is, portions of the neck portion 12 a contacting the insertion 2 are cut under the condition that the limit diameter of the neck portion 12 a is maintained, thereby producing the cutting planes 14. The range of the movement of the femoral stem 1 is expanded in compensation for the cutting of the portions of the neck portion 12a. That is, as shown in FIG. 6, in the flexion of the femoral stem 1, the range of the movement of the femoral stem 1 is increased by approximately 10°.
However, since the acetabulum cup is inserted into the pelvis at an angle of 40˜50° when the artificial coxal articulation is inserted into the pelvis, the acetabulum cap of the femoral stem 1 in the joint movement does not interfere with the front and rear cutting planes 14 of the neck portion 12a, but interferes with corners of the neck portion 12a inwardly turned at an angle of 40˜50°, as shown in FIG. 7. Accordingly, the increase in the range of movement of the artificial coxal articulation is substantially no more than approximately 50.
Compared to the coxal articulation of the human body, which has the angle of flexion of approximately 130˜135°, the above artificial coxal articulation substantially has the angle of flexion of at most 110°. Accordingly, the above discordance generates interference when the flexion of the artificial coxal articulation is performed.
Such interference generates abrasion of an insertion made of polyethylene in the acetabulum cup, breakage of an insertion made of ceramic in the acetabulum cup, dislocation of artificial coxal articulation, or detachment of the artificial coxal articulation. Accordingly, the increase in the range of movement of the artificial coxal articulation is essential, and, particularly, is important in the East requiring eating life while sitting.